A well known method of making plastic pipe is through the use of a traveling tunnel of mold blocks which are fed plastic from an extruder. The plastic is formed into the shape of the pipe in the mold tunnel.
An inherent feature of the plastic is that it shrinks as it cools while moving down the mold tunnel. Some shrinkage can produce benefits such as creating a cooling medium receiving gap between the plastic and the mold blocks which has been taken advantage of by Corma Inc. of Toronto, Canada. However, if there is excessive shrinking then the gap becomes undesirably large.
A conventional moving mold tunnel which is one in which mold blocks circulate around an endless track is not easily adapted to compensate for plastic shrinkage in the mold tunnel. This is because the mold blocks forming the downstream end of the tunnel recirculate back to the upstream end of the tunnel. Accordingly, they must have an internal mold size consistent with the plastic dimension introduced at the upstream end of the tunnel before the plastic has had an opportunity to shrink.